Pressure swing adsorption is a well-known method for the separation of bulk gas mixtures and for the purification of gas streams containing low concentrations of undesirable components. The method has been developed and adapted for a wide range of feed gases, operating conditions, product purity, and product recovery. Many pressure swing adsorption systems utilize two or more adsorber beds operated in a cyclic sequence in order to maintain a constant product flow rate while selected beds undergo sequential steps including adsorption, depressurization, evacuation, purge, pressure equalization, repressurization, and other related steps. Multiple adsorber beds using numerous process steps are required to achieve high purity and/or recovery of valuable gaseous products such as hydrogen, carbon oxides, synthesis gas, light hydrocarbons, and the like. Multiple adsorber beds using these process steps also are used to recover oxygen from air.
Many of these pressure swing adsorption processes operate partially at pressures below atmospheric and are described in the art as vacuum swing adsorption (VSA) or vacuum-pressure swing adsorption (VPSA) processes. In the present specification, pressure swing adsorption (PSA) is used as a generic term to describe all types of cyclic adsorption systems regardless of operating pressure levels.
In PSA process cycles, the gas needed for the purge and repressurization steps is provided by gas obtained during other process steps. Repressurization can be accomplished by using final product gas, intermediate gas obtained by pressure equalization among beds, pressurized feed gas, or combinations thereof. Purge can be provided by intermediate depressurization gas from other beds and/or by final product gas.
PSA plants typically are designed for optimum efficiency at product flow rates and product purities specified by the product gas consumer. In the operation of most PSA plants, however, the product gas requirements of the consumer vary with time, and the PSA system often must operate at turndown conditions to provide product gas at flow rates which are less than the design flow rates. During such turndown periods, it is desirable to minimize the inefficiencies caused by off-design operation of the PSA system in order to minimize the overall cost of the product gas.
Modifications to a PSA process can be made to minimize the economic impact of turndown operation. Such modifications must be compatible with the specific PSA process cycle and should minimize the need for additional equipment for operating at turndown conditions. The invention described below and defined by the claims which follow offers a simple method for the economic operation of a PSA process cycle at such turndown conditions.
The invention relates to a pressure swing adsorption process for the separation of a pressurized feed gas-containing at least one more strongly adsorbable component and at least one less strongly adsorbable component. The method comprises operation of the process during two periodsxe2x80x94a period of design product gas flow and a period of reduced product gas flow or turndown.
During a period of design product gas flow, the process comprises the steps of
(1) introducing the pressurized feed gas into a feed end of an adsorber bed containing one or more solid adsorbents which preferentially adsorb the more strongly adsorbable component and withdrawing from a product end of the adsorber bed a first adsorber effluent gas enriched in the less strongly adsorbable component, wherein the first adsorber effluent gas is utilized as final product gas;
(2) terminating the introduction of the pressurized feed gas into the adsorber bed while withdrawing from the product end of the adsorber bed a second adsorber effluent gas enriched in the less strongly adsorbable component, wherein the pressure in the adsorber bed decreases while the second adsorber effluent gas is withdrawn therefrom, and wherein the second adsorber effluent gas is utilized as additional final product gas;
(3) depressurizing the adsorber bed to a minimum bed pressure by withdrawing additional gas therefrom;
(4) repressurizing the adsorber bed by introducing repressurization gas into the bed, wherein at least a portion of the repressurization gas is provided by pressurized feed gas; and
(5) repeating steps (1) through (4) in a cyclic manner.
During a period of reduced product gas flow, the process utilizes steps (1) through (5) in modified form wherein the duration of step (2) is extended, a first idle period is introduced into step (3), and a second idle period or a modified idle period is introduced into step (4).
The average volumetric flow ratio of the pressurized feed gas to the final product gas during (a) may be essentially equal to the average volumetric flow ratio of the pressurized feed gas to the final product gas during (b). The more strongly adsorbable component may be nitrogen and the less strongly adsorbable component may be oxygen, and the average oxygen concentration in the final product gas during (a) may be essentially equal to the average oxygen concentration in the final product gas during (b).
The adsorber bed may be one of a plurality of adsorber beds, each of which undergoes in turn steps (1) through (5) during (a) and steps (1) through (5) in modified form during (b). In this embodiment, the depressurizing of each adsorber bed in step (3) may be accomplished by
(3a) withdrawing a first gas stream from the bed until the pressure therein reaches a first intermediate pressure and utilizing at least a portion of the first gas stream to purge another bed;
(3b) withdrawing a second gas stream from the bed until the pressure therein reaches a second intermediate pressure and utilizing at least a portion of the second gas stream to pressurize yet another bed; and
(3c) evacuating the bed from the feed end using a vacuum blower until the pressure therein reaches the minimum bed pressure.
The first idle period may follow step (3b). The process may further comprise purging the bed following the evacuation step (3c) by introducing a purge gas into the product end of the bed while continuing to evacuate gas from the feed end of the bed. This purge gas may be provided to the bed by a first gas stream withdrawn from another bed which is in step (3a).
The process may further comprise evacuating the bed from the feed end while withdrawing the second gas stream from the product end of the bed during step (3b). The first idle period may follow step (3b).
The repressurizing of the bed in step (4) may be accomplished by
(4a) introducing repressurizing gas into the adsorber bed wherein a portion of the repressurizing gas is provided by a second gas stream withdrawn from another bed which is in step (3b); and
(4b) introducing repressurizing gas into the adsorber bed wherein a portion of the repressurizing gas is provided by pressurized feed gas from a feed gas compressor.
The second idle period may follow step (4a); alternatively, the modified idle period may follow step (4a). In one embodiment, the pressurized feed gas may be air, wherein the more strongly adsorbable component is nitrogen, the less strongly adsorbable component is oxygen, and the final product gas is enriched in oxygen. In this embodiment, a portion of the repressurization gas in step (4a) may be provided by allowing atmospheric air to flow into the adsorber bed when the pressure in the bed is initially below atmospheric pressure. A portion of the repressurization gas in step (4b) may be provided by allowing atmospheric air to flow into the adsorber bed when the pressure in the bed is initially below atmospheric pressure.
The vacuum blower has an inlet which may be in flow communication with atmospheric air during step (4a). In addition, the feed gas compressor may discharge to the atmosphere during step (4a).
In one specific embodiment, the invention relates to a pressure swing adsorption process for the separation of a pressurized feed gas containing at least one more strongly adsorbable component and at least one less strongly adsorbable component. The method comprises operating the process during two periodsxe2x80x94a period of design product gas flow and a period of reduced product gas flow or turndown.
During a period of design product gas flow, the process comprises the steps of
(1) introducing the pressurized feed gas into a feed end of an adsorber bed containing one or more solid adsorbents which preferentially adsorbs the more strongly adsorbable component and withdrawing from a product end of the adsorber bed a first adsorber effluent gas enriched in the less strongly adsorbable component, wherein the first adsorber effluent gas is utilized as a final product gas, wherein the adsorber bed is one of a plurality of adsorber beds;
(2) terminating the introduction of the pressurized feed gas into the adsorber bed while withdrawing from the product end of the adsorber bed a second adsorber effluent gas enriched in the less strongly adsorbable component, wherein the pressure in the adsorber bed decreases while the second adsorber effluent gas is withdrawn therefrom, and wherein the second adsorber effluent gas is utilized as a final product gas;
(3) depressurizing the adsorber bed to a minimum bed pressure by withdrawing additional gas therefrom, wherein the depressurizing of the adsorber bed is accomplished by
(3a) withdrawing a first gas stream from the bed until the pressure therein reaches a first intermediate pressure and utilizing at least a portion of the first gas stream to purge another bed;
(3b) withdrawing a second gas stream from the bed until the pressure therein reaches a second intermediate pressure and utilizing at least a portion of the second gas stream to pressurize yet another bed; and
(3c) evacuating the bed from the feed end until the pressure therein reaches the minimum bed pressure;
(4) purging the bed following the evacuation of step (3c) by introducing a purge gas into the product end of the bed while continuing to evacuate gas from the feed end of the bed, wherein the purge gas is provided to the bed by a first gas stream withdrawn from another bed in step (3a);
(5) repressurizing the adsorber bed by introducing a second gas stream withdrawn from another bed which is in step (3b);
(6) further repressurizing the adsorber bed by introducing pressurized feed gas into the bed; and
(7) repeating steps (1) through (6) in a cyclic manner.
During a period of reduced product gas flow, the process utilizes steps (1) through (5) in modified form wherein the duration of step (2) is extended, a first idle period is introduced into step (3), and a second idle period or a modified idle period is introduced into step (4).
The pressurized feed gas may be air, wherein the more strongly adsorbable component is nitrogen, the less strongly adsorbable component is oxygen, and the final product gas is enriched in oxygen. The process may further comprise, following step (5) and prior to step (6), allowing atmospheric air to flow into the adsorber bed when the pressure in the bed is initially below atmospheric pressure. The process may further comprise evacuating the bed from the feed end while withdrawing the second gas stream from the product end of the bed during step (3b).
The average volumetric flow ratio of the pressurized feed gas to the final product gas during (a) may be essentially equal to the average volumetric flow ratio of the pressurized feed gas to the final product gas during (b). The more strongly adsorbable component may be nitrogen and the less strongly adsorbable component may be oxygen, and the average oxygen concentration in the final product gas during (a) may be essentially equal to the average oxygen concentration in the final product gas during (b).